1) Field of the Invention
The present invention relates to an apparatus for separating fibers from reject material produced in the manufacture of recycled pulp, the apparatus comprising a rotating reject cylinder provided with openings, to one end of which the reject material is fed and from the other end of which it is discharged, water nozzles for feeding water to the reject material in the cylinder to separate fibers from it, a collecting basin for recovering water and the fibers separated with it, and discharge channels connected with the collecting basin, and means for discharging fibrous water and light and heavy particles from the collecting basin.
2) Description of Related Art
The use of recycled pulp becomes more and more common because of waste matter problems and to save forests. In the production of recycled pulp, among waste material there are various materials that are not desired in the paper and board manufacturing processes. Such undesirable materials include above all metals, cords, glass, textiles, wood, sand and building materials, synthetic materials, “synthetic papers” and other waste material regardless of its quality or origin. These have to be removed from the recycled pulp before the pulp is used in further processes.
The biggest impurities are separated from the recycled pulp when the incoming pulp material is pulped. Furthermore, sizeable waste which passes with the pulped recycled pulp is removed in the coarse screening or high-consistency or low-consistency refiners. The reject pulp produced in these stages is led to a separate fiber recovery apparatus, in which the reject pulp is washed in order to separate and recover the fibers that still exist with it and are advantageous in the further processes. Generally a fiber recovery apparatus has a cylindrical drum provided with holes or openings, into which the reject pulp is fed. Dilution or washing water required for washing the reject pulp is fed partly into the drum and partly outside of it, whereupon the water takes fibers from the reject pulp with it and they are discharged to a collecting basin under the holes of the drum. In known solutions the washing or cleaning drum of the reject pulp rotates either in the air or partly under the surface of the washing and dilution water. These devices typically comprise either one washing drum or two drums with different diameters. In many known screening drums with one drum, the drum is divided into two or more successive screening phases, wherein the screening phases are separated with different separation walls into successive sections in order to control the residence time in different phases. In the solutions, the reject pulp to be screened is fed to the casing part on the feed side, which can have holes or no holes, and the reject pulp is discharged from the casing part containing holes on the discharge side after the separation wall. Such solutions are disclosed, for example, in Finnish Patents 63075 and 66441. However, the operating power of these apparatuses is quite weak and also the screening is fairly slow, especially if the casing part on the feed side of the apparatus does not have holes, since in that case the fibers cannot be washed away from the rest of the reject pulp efficiently.
Finnish Patent 94263 and U.S. Pat. No. 1,183,298 disclose, however, apparatuses for washing reject pulp, comprising two coaxial screen surfaces rotating simultaneously, whereby the reject pulp is fed to a first screen cylinder having a larger diameter and forming a washing section, the screen cylinder being constantly partly below the fluid level of the collecting basin under it, so that the water in the collecting basin can pass in and out through the holes of the cylinder surface. In the washing stage, not only fibrous material but also light and heavy reject material having a small particle size, such as sand which comes with the reject pulp to be fed into the cylinder, are extracted from the reject pulp into the collecting basin through the holes of the screen cylinder. A problem with these known solutions is that the screen cylinder rotating partly below the fluid level in the collecting basin causes a strong turbulence in the suspension in the basin, consisting of fibrous water and small, light and heavy reject material, whereby the heavy reject particles are prevented from landing on the bottom of the collecting basin and the removal thereof from the washing process becomes harder. As a result, the heavy reject particles remain to circulate in the washing process, thus causing mechanical wear of the apparatuses and an increasing need for stopping the process because of the necessary service operations.